Several chemical reactions occur in a selective catalytic reduction (SCR) system using NH3 as reductant, all of which represent desirable reactions which reduce NOx to elemental nitrogen. The dominant reaction mechanism is represented in equation (1).4NO+4NH3+O2→4N2+6H2O  (1)
Competing, non-selective reactions with oxygen can produce secondary emissions or may unproductively consume NH3. One such non-selective reaction is the complete oxidation of NH3, represented in equation (2).4NH3+5O2→4NO+6H2O  (2)
Furthermore, the reaction of NO2 present in the NO with NH3 is considered to proceed according to reaction (3).3NO2+4NH3→(7/2)N2+6H2O  (3)
Further, the reaction between NH3 and NO and NO2 is represented by reaction (4):NO+NO2+2NH3→2N2+3H2O  (4)
Although the reaction rates of the reactions (1), (3) and (4) vary greatly depending on the reaction temperature and the sort of the catalyst used, that of the reaction (4) is, in general, 2 to 10 times as high as those of the reactions (1) and (3).
The application of SCR technology to treat NOx emissions from vehicular IC engines, particularly lean-burn IC engines, is well known. A typical prior art SCR catalyst disclosed for this purpose includes V2O5/WO3 supported on TiO2 (see WO 99/39809). However, in some applications the thermal durability and performance of vanadium-based catalyst may not be acceptable.
One class of SCR catalysts that has been investigated for treating NOx from internal combustion engine exhaust gas is transition metal exchanged zeolites (see WO 99/39809 and U.S. Pat. No. 4,961,917). However, in use, certain aluminosilicate zeolites such as ZSM-5 and beta zeolites have a number of drawbacks. They are susceptible to dealumination during high temperature hydrothermal ageing resulting in a loss of acidity, especially with Cu/beta and Cu/ZSM-5 catalysts; both beta- and ZSM-5-based catalysts are also affected by hydrocarbons which become adsorbed on the catalysts at relatively low temperatures and are oxidised as the temperature of the catalytic system is raised generating a significant exotherm, which can thermally damage the catalyst. This problem is particularly acute in vehicular diesel applications where significant quantities of hydrocarbon can be adsorbed on the catalyst during cold-start; and beta and ZSM-5 zeolites are also prone to coking by hydrocarbons, which reduces catalyst performance. Accordingly, we have directed research to finding alternatives to transition metal exchanged zeolites and vanadium-based catalysts for SCR.
U.S. Pat. No. 5,552,128 claims a method for converting nitrogen oxides to nitrogen by contacting the nitrogen oxides with a reducing agent in the presence of a catalyst consisting essentially of an acidic solid component comprising a Group IVB metal oxide modified with an oxyanion of a Group VIB metal and further containing at least one metal selected from the group consisting of Group IB, Group IVA, Group VB, Group VIIB and Group VIII and mixtures thereof. The catalysts can be prepared by impregnation, co-precipitation or hydrothermal treatment of a hydrated Group IVB metal prior to contact with a Group VIB metal. A preferred catalyst consists essentially of iron (Group VIII), tungsten (Group VIB) and zirconium (Group IVB). Although a catalyst consisting of zirconium, tungsten and cerium is exemplified (Catalyst B), our understanding of the prosecution file is that cerium, and rare earth metals more generally, were dropped from the claims, and the claims were restricted from “comprising” to “consisting essentially of”, in order to meet an objection by the Examiner based on Japanese patent publication no. 6-190276.
Japanese patent publication no. 6-190276 discloses a catalyst for selectively reducing NOx with hydrocarbons in a comparatively low-temperature region, which catalyst comprises both a basic metal (such as magnesium, calcium, strontium, barium, sodium, potassium, rubidium, caesium, lanthanum or zinc) or its oxide and an acidic metal (such as tungsten, molybdenum, cobalt, iron, silver or silicon) or its oxide supported on aluminium oxide (Al2O3), zirconium oxide (ZrO2), yttrium oxide (Y2O3), potassium oxide (Ga2O3) or tin oxide (SnO2) which reduce the nitrogen oxide by the selective reduction method to nitrogen by being brought into contact with the nitrogen oxide together with the hydrocarbon as the reducing gas. Illustrative examples include gamma aluminium oxide supporting both tungsten oxide or molybdenum oxide and magnesium oxide and zirconium oxide supporting both tungsten oxide and magnesium oxide.
EP 1736232 discloses a catalyst system comprising a first reaction unit which is loaded with a first catalyst containing, as active constituents, a composite oxide consisting of two or more oxides selected from silica, alumina, titania, zirconia and tungsten oxide, and a rare earth metal or a transition metal (excluding Cu, Co, Ni, Mn, Cr and V), and a second reaction unit which is loaded with a second catalyst containing, as active constituents, a noble metal and a silica-alumina composite oxide. Illustrative examples of the first catalyst include the composite oxides Ce—Ti—SO4—Zr (obtained by adding cerium and sulfur to a titania-zirconia type complex oxide), Fe—Si—Al (obtained by adding iron to a silica-alumina type complex oxide) and Ce—W—Zr (obtained by adding cerium to a tungsten oxide-zirconia type complex oxide).
U.S. Pat. No. 4,085,193 discloses a catalyst composition for reducing nitrogen oxides comprising an intimate mixture of titanium as component A with at least one metal selected from the group consisting of molybdenum (Mo), tungsten (W), iron (Fe), vanadium, (V), nickel (Ni), cobalt (Co), copper (Cu), chromium (Cr) and uranium (U) as component B, in the form of their oxides, and a process for reducing nitrogen oxides to nitrogen, which comprises contacting a gaseous mixture containing nitrogen oxides and molecular oxygen and a reducing gas with the catalyst composition at an elevated temperature. Ti—W and Ti—W—Fe are illustrated and the activity of Ti—W is compared favourably with the activity of Zr—W.
U.S. Pat. No. 4,916,107 discloses a catalyst for the selective reduction with ammonia of nitrogen oxides from an intimate mixture of at least three metals in the form of their oxides, namely (A) titanium as constituent (A), (B1) tungsten as the first constituent B, and (B2) at least one of the metals vanadium, iron, niobium, and/or molybdenum as the second constituent (B), with an atomic ratio of the metals of constituent (A) to (B) of 1:0.001 to 1, preferably 1:0.003 to 0.3.
JP 52-42464 discloses a method of reducing and removing NOx in exhaust gas comprising contacting the exhaust gas and ammonia with a catalyst in a temperature range of 200-500° C., said catalyst containing 50-97% (atomic percent) titanium oxide as its first active ingredient, 2-49% (atomic percent) cerium oxide as its second active ingredient, and 1-30% (atomic percent) of at least one compound selected from molybdenum oxide, tungsten oxide, vanadium oxide, iron oxide, and copper oxide as its third active ingredient. Illustrative examples include Ti—Ce—Cu, Ti—Ce—Fe, Ti—Ce—W and Ti—Ce—Mo.
GB 1473883 discloses a catalyst composition for the reduction of nitrogen oxides comprising iron and tungsten in the form of their oxides in an atomic ratio Fe/W of 1:0.001-1 and having a surface area of at least 5 m2/g obtainable by calcining at 300-700° C. The catalyst may contain an oxide of a further element from Groups IB, IIA, IIIB, IV, VA, VIA, VIII or of the rare earths, e.g. Cu, Mg, Al, Si, Ti, Zr, Sn, V, Nb, Cr, Mo, Co, Ni and Ce, in an atomic ratio based on iron not exceeding 1:0.15. The catalyst may be supported, e.g. on silica, alumina, silica-alumina, diatomaceous earth, acid clay, active clay, zeolite, titania or zirconia and may be prepared by impregnation or precipitation.
N. Apostolescu et al. (Applied Catalysis B: Environmental 62 (2006) 104-114) disclose a SCR catalyst for treating NOx in diesel exhaust gas obtainable by coating ZrO2 with 1.4 mol % Fe and 7.0 mol % WO3 SCR catalyst which demonstrates improved SCR performance relative to Fe2O3/ZrO2. The ZrO2 is obtained by adding ZrO(NO3)2 to an aqueous solution of hydrazine. In our own investigations, we have determined that for improved thermal stability and SCR activity it is important for the ZrO2 to be present in its tetragonal phase. We have investigated the N. Apostolescu et al. catalysts and have found that, whilst they claim to obtain ZrO2 tetragonal phase, their catalyst is not as active as catalysts containing ZrO2 that we have prepared.
JP 2003-326167 discloses a SCR catalyst suitable for treating NOx in exhaust gas from an internal combustion engine comprising tungsten oxide or molybdenum oxide on a carrier consisting of sulphated zirconium oxide.
SAE 2007-01-0238 discloses investigations into acidic doped zirconia for use in NH3-SCR catalysis. The materials tested include Zr—Si, Zr—Si—W and Zr—Ti—Si—W.